[TIP] Catalyst / fglrx and DVI to HDMI adapters (audio)

[Rafał Miłecki]

For the last few days it was I was testing fglrx on various cards to
trace HDMI setup. One thing that was killing me was no HDMI audio when
using some generic/cheap DVI to HDMI adapter with my HD 4850.

I started digging and it has appeared to be common problem when using
Catalyst / fglrx. Windows users were also experiencing this problem,
without any known workaround.

I've found one explanation by droidhacker:
http://phoronix.com/forums/showthread.php?23451-HDMI-Audio-with-fglrx&p=124891#post124891
> If that is the case, you need to be aware that AMD sells a couple of PROPRIETARY adapter plugs, which fglrx can DETECT in order to decide whether or not to enable HDMI audio.

That sounded weird, I didn't believe that. There wasn't any
explanation how it's handled, I couldn't imagine how fglrx can "talk"
to the simple adapter. Overall it should just map the PINs, not
include any logic!

But today I've found another discussion:
http://www.avsforum.com/t/1080627/atis-magic-dvi-to-hdmi-dongle

That finally explains what is happening. So it appears that ATI's
proprietary adapter include some tiny I2C protocol that allows
identifying them! It seems that Catalyst / fglrx uses some simple I2C
talk to check if the adapter is made by ATI and allows or refuses to
use HDMI mode.
That also explains why radeon driver never had any problems with audio
over DVI, no matter what adapter was used.

Another ATI/fgllx mystery explained ;)

-- 
Rafał